# Zähler

Bibliothek: | Memory
Eingeführt: | 2.3.0
Aussehen: |

## Verhalten

The counter holds a single value, whose value is emitted on the output Q. Each time the clock input (diagrammed with a triangle on the component's south edge) triggers according to its Trigger attribute, the value in the counter may update based on the two inputs on the component's west edge: The upper input is called load and the lower is called count, and they are interpreted as follows.

load | count | trigger action
0 or z | 0 | The counter remains unchanged.
0 or z | 1 or z | The counter increments.
1 | 0 | The counter loads the value found at the D input.
1 | 1 or z | The counter decrements.

The range of counting can be configured using the Maximum Value attribute. When the counter reaches this value, the next increment wraps the counter back to 0; and if it is at 0, then a decrement will wrap the counter around back to its maximum value.

In addition to the output Q, the component also includes a single-bit output carry. This is 1 whenever the counter is at its maximum and the load and count inputs indicate that the component should increment on the next step - or when the counter is at 0 and the load and count inputs indicate to decrement at the next step.

The clear input resets the counter's value to 0 (all zeroes) asynchronously; that is, as long as the clr input is 1, the value is pinned to 0, regardless of the clock input.

## Pins

East edge, labeled Q (output, bit width matches Data Bits attribute)
  Outputs the value currently stored by the counter.
Ostseite, unterer Pin (Ausgang, Bitbreite 1)
  Carry: When load and count indicate to increment, this output is 1 whenever the counter is at its maximum. When load and count indicate to decrement, this output is 1 whenever the counter is at 0. At all other times, this output is 0.
West edge, top pin (input, bit width 1)
  Load: When this is 1 while the count input is 0, the counter will load the value found at the data input at the next clock trigger - or, if the count input happens to be 1, the counter's value will decrement.
West edge, middle pin labeled D (input, bit with matches Data Bits attribute)
  Data: When the clock triggers while load is 1 and count is 0, the counter's value changes to the value found at this input.
West edge, lower pin labeled ct (input, bit width 1)
  Count: When this is 1 or unconnected, the value in the counter increments whenever the clock input is triggered - or it decrements if the load input happens to also be 1.
South edge, indicated with a triangle (input, bit width 1)
  Clock: At the instant that this is triggered as specified by the Trigger attribute, the counter updates as indicated by the load and count inputs.
South edge, labeled 0 (input, bit width 1)
  Clear: When 0 or undefined, this input has no effect. As long as it is 1, the counter's value is asynchronously pinned to 0. This occurs asynchronously - that is, without regard to the current clock input value. As long as this is 1, the other inputs have no effect.

## Logisim-evolution Appearance (IEEE/IEC Standard)

When the Appearance attribute is set to Logisim-evolution, the counter uses standard IEC/IEEE graphical symbols (IEC 617-12 and IEEE Std 91-1984). This appearance includes additional control pins and mode indicators:

Symbol | Label | Function
--- | --- | ---
G (or G5) | Count Enable | When high (1), counting is enabled. When low (0), the count is held regardless of clock pulses.
M1 | [load] | Load mode control. Activates loading from the Data input.
M2 | [count] | Count mode control. Activates counting operation.
M3 | [up] | Up-count control. Selects increment mode.
M4 | [down] | Down-count control. Selects decrement mode.
2,3,5+/C6 | Clock+ | Clock input for specific mode combinations (with count enable active).
2,4,5- | Clock- | Alternative clock input path for different operational modes.

Note: The Logisim-evolution appearance may show duplicate clock and carry pins. These represent different operational modes of the counter according to the IEC/IEEE standard. The functionality is identical to the Classic appearance; only the graphical representation differs.

## Attribute

When the component is selected or being added, Alt-0 through Alt-9 alter its Data Bits attribute.

Data Bits
  The bit width of the value emitted by the component.
Maximum Value
  The maximum value, at which point the counter will set its carry output.
Action On Overflow
  The behavior when the counter attempts to increment beyond the maximum value or decrement beyond 0. Four possible actions are supported:
  Wrap around
    The next value is 0 (if incrementing - the maximum value if decrementing)
  Stay at value
    The counter's value remains at the maximum (or 0 if decrementing)
  Continue counting
    The counter continues incrementing/decrementing, keeping the number of bits as provided by the Data Bits attribute
  Load next value
    The next value is loaded from the D input.
Trigger
  Configures how the clock input is interpreted. The value rising edge indicates that the counter should update its value at the instant when the clock rises from 0 to 1. The falling edge value indicates that it should update at the instant the clock falls from 1 to 0.
Label
  The text within the label associated with the component.
Label Font
  The font with which to render the label.

## Verhalten des Schaltwerkzeugs

Clicking the counter brings keyboard focus to the component (indicated by a red rectangle), and typing hexadecimal digits will change the value stored in the counter.

## Verhalten des Textwerkzeugs

Allows the label associated with the component to be edited.

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